1. Field of the Invention
The present invention relates to a method for removing contaminants from a hydrocarbon feed stream to an etherification reactor. More particularly, the invention relates to a process for nitriles from hydrocarbon feed streams.
2. Related Information
Since the Clean Air Act Amendments of 1990 refiners have searched for ways to introduce oxygen into gasoline to produce cleaner burning reformulated fuels. In addition to methyl tertiary butyl ether (MTBE), other suitable ethers for this purpose are tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE).
Manufacturers are looking for alternative sources of hydrocarbons for the production of ethers and oxygenates. These sources include C.sub.5 hydrocarbons which contain isoamylenes that are suitable for the production of TAME.
TAME is formed by the reaction of isoamylene and methanol at mild operating conditions over an acid catalyst. The selectivity of this reaction is limited by equilibrium constraints. By using a catalytic distillation column, essentially complete conversion is attainable.
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873 all commonly assigned herewith.
Briefly the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure. As embodied in the etherification of isoamylene's the olefin and an excess of methanol are first fed to a fixed bed reactor wherein most of the olefin is reacted to form the corresponding ether, tertiary amyl methyl ether (TAME). The fixed bed reactor is operated at a given pressure such that the reaction mixture is at the boiling point, thereby removing the exothermic heat of reaction by vaporization of the mixture. The fixed bed reactor and process are described more completely in U.S. Pat. No. 4,950,803 which is hereby incorporated by reference.
The effluent from the fixed bed reactor is then fed to the distillation column reactor wherein the remainder of the isoamylenes are converted to the ether and the methanol is separated from the ether which is withdrawn as bottoms. The C.sub.5 olefin stream generally contains only about 10 to 60 percent olefin, the remainder being inerts which are removed in the overheads from the distillation column reactor. The overhead hydrocarbon raffinate contains an amount of methanol up to the azeotropic concentration of about 12 wt %.
This stream is washed with water to separate methanol from the hydrocarbons. Methanol is then distilled to remove the water. In this invention a portion of the water-methanol extract is first used as a solvent to extract nitriles from the C.sub.5 feed before it is sent to a distillation tower.
The etherification processes typically utilize strongly acidic ion exchange resins as etherification catalysts, which are strongly acidic organic polymers. As an isobutylene or isoamylene molecule meets alcohol at an active site, the reaction takes place rapidly forming ether.
The activity of the catalyst for etherification reactions is a function of the acid loading or capacity of the resin. This functionality is not linear; a loss of 20% of acid sites on the catalyst gives approximately 50% loss of activity for conversion to ether. It is therefore important to minimize the deactivation of the catalyst with effective feed pretreatment to maintain peak performance and long catalyst life. The loss of catalytic activity may be caused by the adsorption of basic compounds or metal ions, the blockage of the active sites by polymeric products, or by the splitting off the functional groups due to long term operation at temperatures above 240.degree. F. The latter two causes are affected by the operating conditions of the etherification reactor. The major source of lost activity is typically from poisons entering with the feedstocks to the unit. Poisons to the catalyst include basic compounds such as ammonia, amines, caustic soda, and nitriles. In particular, acetonitrile (ACN) and propionitrile (PN) have been found to deactivate the catalyst.
In refinery applications, the largest source of hydrocarbon feedstock containing isoolefins is the stream from the fluidized catalytic cracking unit (FCCU). Some C.sub.4 and C.sub.5 's are also obtained from fluid or delayed cokers. Nitriles are formed in these units that enters the etherification process with the hydrocarbon feed stream. The amount of nitriles in the feed varies with the severity of the catalytic cracker operation, crude source, and catalyst used in the FCCU. Propionitrile has been found to be a particular problem in the C.sub.5 stream. Unlike all the other feed poisons which deactivate the catalyst in a plug flow fashion through the catalyst bed, nitrile's deactivation mechanism is not immediate and results in a diffused deactivation throughout the entire bed. No effective means has been found to reactivate the catalyst.
In order to obtain adequate run lengths with the catalyst and optimum performance, the first step in the etherification process is a feed pretreatment step designed to remove the poisons to very low levels (&lt;1 ppm). Since the poisons are much more soluble in water than hydrocarbon, the common treatment is a multistage water wash either in countercurrent or concurrent systems. The water and hydrocarbon streams are contacted utilizing trays or packing. In the countercurrent tower the continuous water phase flows down the column as the liquid hydrocarbon droplets are dispersed upwards. The design variables include the number of theoretical contact stages and the flow rate of water.
Normally, nitriles are removed from C.sub.4 and C.sub.5 hydrocarbon streams by washing the hydrocarbon stream with water to extract the nitriles into the water phase. This works well with acetonitrile but is much less effective with propionitrile.
It is an advantage of the present invention to provide an improved method for the removal of nitriles from hydrocarbon streams, in particular C.sub.5 streams. It is a particular feature of the present process that propionitrile removal from the C.sub.5 streams is enhanced.